In production lines of vehicles, vehicle bodies are transported using carts, and parts are assembled to the vehicle bodies which are being transported, manually or using machines. A method of transporting a vehicle body for this purpose is known, e.g., in Patent Document 1.
FIG. 19 shows a vehicle production line disclosed in Patent Document 1. Referring to FIG. 19, a plurality of carts 202 are provided in a closed first transportation path 201. A vehicle body 203 is placed on each of the carts 202.
In the drawing, at a point P1, serving carts 204, 205 for supplying (distributing) parts are coupled to front and rear portions of the cart 202. Parts to be assembled to the vehicle body 203 are placed on these serving carts 204, 205. During movement of the cart 202 from the point P1 to a point P2, the parts are assembled to the vehicle body 203. After the service carts 204, 205 become vacant, at the point P2, the service carts 204, 205 are removed from the cart 202.
At a point P3, the vehicle body 203 is picked up from the cart 202 using a workpiece transfer apparatus 216 such as a lifting apparatus. After the cart 202 becomes vacant, the cart 202 moves along the first transportation path 201 counterclockwise in the drawing, and returns to the point P1. Some of the parts are partially assembled in the first transportation path 201, and the remaining parts are assembled in a second transportation path 211.
A plurality of carts 212 are provided also in the second transportation path 211. At a point P4, serving carts 214, 215 are coupled to front and rear portions of the cart 212. Then, at the point P4, the vehicle body 203 is placed on the cart 212 by the workpiece transfer apparatus 216. During movement from the point P4 to a point P5, parts (remaining parts) are assembled to the vehicle body 203.
That is, the parts from the serving cart 214 are assembled to the front portion of the vehicle body 203, and the parts from the serving cart 215 are assembled to the rear portion of the vehicle body 203. Since the center of the vehicle body 203 is far from the serving carts 214, 215, the flow line of the operator becomes long. Therefore, while there is a need to improve the productivity, in the case of assembling parts at the center of the vehicle body 203, it is desirable to shorten the flow line of the operator.
Further, each of the carts 202, 212 is equipped with a drive source, and can travel by itself using the own drive source. At the point P3, in the state where the cart 202 is stopped, the vehicle body 203 is picked up therefrom. At the point P4, in the state where the cart 212 is stopped, the vehicle body 203 is placed on the cart 212. In the meanwhile, the other carts 202, 212 move at low speed. In order to avoid collisions between the carts 202, 212, a certain interval needs to be kept between the adjacent carts 202, and likewise, a certain interval needs to be kept between the adjacent carts 212.
For this purpose, in order to improve the productivity for certain lengths of the transportation paths 201, 211, it is desirable to position the carts 202, 212 more densely.